1. Field of the Invention
The present invention relates to a battery pack and a charge control method used for the battery pack, and more particularly, to a method of controlling charging and discharging of a battery pack having battery cells using a positive electrode active material with an olivine crystal structure.
2. Description of the Related Art
In recent years, a battery pack using a lithium ion secondary battery has widely been used as a power source in a portable electronic apparatus such as a laptop personal computer, a cellular phone, and a PDA (Personal Digital Assistant). The lithium ion secondary battery has advantages such as lightness, high capacity, ease of residual capacity detection, and long life cycle.
In a lithium ion secondary battery, generally, charging is performed by a constant current and constant voltage method and the charging ends at current termination at which the fact that a current value is lowered up to a predetermined current value is detected by a charge time or a constant voltage region.
When the lithium ion secondary battery using a cobalt-based, manganese-based, or nickel-based compound oxide particle as a positive electrode active material according to the related art is charged by the constant current and constant voltage, a charge characteristic is shown in FIG. 1. When the constant current charging is performed, the voltage of the battery gradually increases, and then the current value gradually decreases when the voltage of the battery reaches a predetermined voltage. When a method of detecting the current value and terminating the charging is used, the capacity of the battery can be sufficiently used.
When the charging is terminated at a predetermined charge time, there is a possibility that the battery is exposed to a high voltage state for a long time and thus the life of the battery is shortened. When a battery pack in which a plurality of lithium ion secondary batteries is connected to each other in series is charged, a general charger charges the battery pack with a given output voltage. Therefore, a voltage balance of the respective battery may be lost. In this case, the charging is usually stopped by an overcharge protection function.
On the other hand, a battery pack is charged not by the above-mentioned constant current and constant voltage charging method but by a constant current method in order to increase the life of the battery or improve safety of overcharging. FIG. 2 shows a charge characteristic when the lithium ion secondary battery using a cobalt-based, manganese-based, or nickel-based compound oxide particle used from the past as a positive electrode active material is charged by the constant current charging. However, the constant current charging method has a problem with the charging characteristic of a battery in that a charging ratio is just about 80% of the entire capacity of the battery. In the case of the constant current charging, when the voltage of the battery reaches a predetermined voltage, the charging is terminated. Therefore, the charge time is short, while the charge capacity is smaller. Therefore, the performance of the battery may not be realized.
When the plurality of secondary batteries in which the variation in a voltage or a difference in a deterioration degree occurs is used, it is difficult to control charging and discharging and thus a burden is imposed on the secondary batteries. For example, although there are many secondary batteries of which a voltage does not reach a discharge termination voltage, the discharging may be stopped. When the secondary batteries are charged, a problem may arise in that the charging is stopped even as some secondary batteries are not fully charged. Thus, as the batteries gradually deteriorate, a usable time (that is, a discharge time) of the battery pack decreases. Moreover, since the time necessary to fully charge a battery pack and the number of charging and discharging increase, the secondary batteries may further deteriorate.
On the other hand, even when the voltage of a certain secondary battery reaches a discharge termination voltage or a certain secondary battery is fully charged, a problem may arise in that charging and discharging may continue due to the fact that the voltage of another secondary battery is a chargeable or dischargeable voltage.
In order to resolve these problems, for example, Japanese Patent No. 4213624 and Japanese Patent No. 3951068 disclose techniques for keeping a balance between secondary batteries by temporarily discharging only the secondary batteries necessary to be discharged.